The main problem with RTFC is that it tells you nothing about how the code is supposed to behave, which is important. Yes, I can see, by looking at the source,

The main problem with RTFC is that it tells you nothing about how the code is supposed to behave, which is important. Yes, I can see, by looking at the source, how this version of this code behaves, but unless I have an interface spec or some documentation, I have no idea whether this behavior is intended or not, and therefore I have no idea whether the behavior is something I can count on in v.next. This is one reason my current project uses a whole stable of ruby gems that we are absolutely terrified of ever upgrading: all we know about their behavior is what we can observe about one specific version of the source.